MUST-BE-EQUAL

The form (must-be-equal logic exec) evaluates to logic in the ACL2
logic but evaluates to exec in raw Lisp. The point is to be able to
write one definition to reason about logically but another for evaluation.
Please see mbe and see mbt for appropriate macros to use, rather than
calling must-be-equal directly, since it is easy to commute the arguments
of must-be-equal by accident.